There are a number of electronic identification methods and devices presently in use. Everyone is familiar with the ubiquitous bar codes and magnetic strips that, together with their readers, are widely employed by businesses and others. An inherent limitation of bar codes and magnetic strip is the effective range at which they can be read, which is quite short. Magnetic strips, for example, generally require the reader to be in direct contact with the strip in order to detect and decode any data. In the very few cases where a magnetic strip is read with a device other than a direct contact reader, the effective reading range is still only a few centimeters at best. Similarly, the effective range at which bar codes can be reliably read is typically no better than a few centimeters. Because the read range for bar codes and magnetic strip is so short, they are usually read one at a time and seldom does one bar code or magnetic strip interfere with another.
Another prior art identification device is the radio frequency identification (“RFID”) tag. When interrogated, RFID tags reflect or retransmit a radio frequency signal that returns an encoded identification number, such as the RFID tags used to collect highway and bridge tolls. Although prior art RFID tags based on a chip have a longer reliable range than magnetic strips or bar codes, they are generally so expensive that they are not widely used, which means that each prior art RFID tag is generally individually read. In short, very little opportunity exists for prior art RFID tags to interfere with one another.
With the introduction of inexpensive identification tags based on surface acoustic wave (SAW) technology that can be read at a relatively long range, certain interference problems must now be addressed. Although there will not be a problem where SAW tags are individually read or simultaneously read in small groups, such will not be the case where a large number of SAW tags are being simultaneously interrogated. Such would be the case, for example, where a large number of articles are stacked on a pallet with each article bearing its own globally unique SAW tag. The large number of responses to an interrogation pulse would make it difficult for the SAW tag reader to accurately detect and decode each response in order to reliably identify each article on the pallet. This type of code collision problem as well as the inter-symbol interference problem caused by so many responses being transmitted at one time needs to be addressed before the full potential using SAW identification tags can be realized.
Accordingly, what is needed in the art is a system for focusing an interrogation pulse to within a defined space so that a SAW identification tag reader can discriminate between coded responses returned from SAW tags located within that space.